The present invention relates to an imaging apparatus like a so-called digital camera in which an imaging element or the like is driven by a power supply such as a battery and so on, and in particular, relates to an imaging apparatus in which a drive power supply system of the imaging element has been improved.
An imaging apparatus, such as a so-called digital camera, in which an image of a photographic subject is imaged by a solid-state imaging element, for example, a CMOS (complementary metal-oxide semiconductor) imaging element or a CCD (charge-coupled device) imaging element, and image data of a still image or a moving image of the photographic subject is obtained and digitally recorded in a recording medium such as memory and so on, is widely used. In such an imaging apparatus, a battery is generally used as a power supply, and a battery voltage is converted to a required voltage by a DC-DC converter of a switching regulator type and so on, and each part of the apparatus is driven. Therefore, in order to effectively use a battery as a power supply over a long term, it is preferable to achieve power saving. In addition, as for an imaging apparatus, high sensitivity and a high picture quality have been demanded, and in order to obtain a favorable shot image quality with less noise even in the case of high sensitivity, an imaging element which is hardly affected by noise has been also demanded even in the case of high sensitivity.
Conventionally, there are techniques to provide power saving and low noise of an imaging apparatus and so on, which are disclosed in Japanese patent publication numbers 2008-99370, 2007-221205, and 2001-339630, for example.
In Japanese patent publication number 2008-99370, a battery device using a power supply circuit with low noise and small power consumption is disclosed. In Japanese patent publication number 2008-99370, in a case where a voltage VDD is higher than a predetermined voltage, a configuration with a charge pump circuit which operates in a step down ratio=2 and steps down the voltage VDD and outputs an intermediate voltage is disclosed. Since the voltage VDD is stepped down by the above charge pump circuit, the intermediate voltage becomes approximately half of that in a case where the intermediate voltage is not stepped down, and power consumption is greatly reduced by an MOS transistor. Due to such a configuration, it is possible to inhibit an increase of power consumption by the voltage VDD with a high voltage, and this inhibition of power consumption makes it possible to minimize or eliminate a heat dissipating section. That is, Japanese patent publication number 2008-99370 discloses that a threshold value is set for a power supply voltage, and a step-down operation of the power supply voltage by the charge pump circuit is controlled, and a power saving operation is performed, and which makes the life of a battery as a power supply longer and inhibits noise due to the power supply.
In Japanese patent publication number 2007-221205, an imaging apparatus in which power consumption at the start of operation of the imaging apparatus is reduced so as not to interfere with a shooting, and which is capable of performing a long-time shooting by a battery is disclosed. Japanese patent publication number 2007-221205 discloses that an imaging apparatus, including at least two circuit units, and a power supply unit which supplies electric power individually to the circuit units, has a start-up mode judging section which judges a start-up mode based on a usage state of the imaging apparatus, and a power supply controlling section which controls an electric power supply from the power supply unit to the circuit units, and the start-up mode judging section judges at least two start-up modes depending on a usage state of the imaging apparatus when shooting. That is, Japanese patent publication number 2007-221205 discloses that at least two start-up modes are determined by the start-up mode judging section depending on a usage state of the imaging apparatus when shooting, and following the procedure by the start-up mode judging section, an electric power supply from the power supply unit to the circuit units is controlled by the power supply controlling section, and thereby power saving is performed, which increases the life of a battery.
Japanese patent publication number 2001-339630 discloses an imaging apparatus which effectively uses a battery by setting or permitting a proper operation mode depending on a remaining amount of battery power. Japanese patent publication number 2001-339630 discloses that an imaging apparatus capable of being driven by a battery predicts power consumption in various operation modes, and judges whether the battery is capable of supplying enough electric power to perform various operation modes or not based on the predicted power consumption, and then executes or permits an operation mode which has been judged that the battery is capable of supplying enough electric power to perform the operation mode. That is, Japanese patent publication number 2001-339630 discloses that by predicting power consumption per operation mode and changing an operation method, power saving is performed, which increases the life of a battery.
[Obvious Appearance of Noise Due to High Sensitivity]
By the way, an imaging apparatus using an imaging element like a so-called digital camera and so on is capable of adjusting and setting the sensitivity of the imaging element by a gain adjustment of the imaging element, and in many cases, it is possible to adjust and set the sensitivity as ISO (international organization for standardization) sensitivity corresponding to ISO sensitivity of a traditional silver halide film, for example. As for ISO sensitivity, for example, there typically are such values as ISO100, ISO200, ISO400, ISO800, ISO1600, and so on, and ISO100 is taken as a reference, and values which increase in geometric progression such as 200, 400, 800, 1600, and etc. are used. For example, at ISO200, shooting with a half amount of light at ISO100 is possible, because the light-receiving sensitivity is twice the ISO100.
As a value of ISO sensitivity increases, light-receiving sensitivity increases; therefore it is possible to shoot a dark photographic subject, and make a shutter speed faster, and shoot a photographic subject moving at high-speed with less shake. On the other hand, increasing the light-receiving sensitivity by an increase of the value of ISO sensitivity makes a gain of the imaging element higher; therefore noise in an image to be imaged can be highly visible. Accordingly, a small noise component which is negligible at low ISO sensitivity appears prominently in an image as an imaging result, when high ISO sensitivity is selected. As such noise components, there are fixed pattern noise, random noise, power supply noise (noise occurring in power supply affects an internal circuit and the like of an imaging element and appears as noise in an imaged image.) and so on.
[Noise Contamination Accompanied with Miniaturization]
In recent years, miniaturization of an imaging apparatus, for example, making a digital camera smaller and thinner, has advanced. Therefore, miniaturization of the imaging apparatus is performed by reducing a size of a component used inside the imaging apparatus, and encouraging a commonalization of a circuit in order to make the imaging apparatus smaller. As for commonalizing a circuit, for example, there are commonalizing a ground pattern (GND) in an analog circuit and in a digital circuit, commonalizing a power supply of each part, and so on. Due to performing a commonalization of a power supply and a commonalization of a ground pattern as described above, a noise current occurring in a circuit may flow to a different circuit via the commonalized ground pattern and power supply. Thus, generally, noise due to noise contamination occurring in a different circuit may appear in a power supply line.
[Shooting Amount/Battery]
As one of the important factors of an imaging apparatus such as a digital camera, there is a shooting amount (imaging amount), that is, the number of shootings (the number of shot images) in the case of a still image, or a shooting time (imaging time) in the case of a moving image. This shooting amount is an important factor next to an imaging quality regarding a specification of an imaging apparatus, and as for an imaging apparatus, obtaining a high picture quality and a great number of shootings at the same time is a main theme. As described above, since miniaturization of an imaging apparatus has been demanded recently, the size of a battery also has become smaller, and therefore it is not easy to increase the capacity of a battery. In addition, as for an imaging apparatus, there is a tendency of an increase of power consumption due to an increase of the number of imaging pixels, an increase of a size of a liquid crystal display device, a complex and a high-speed flash emission and lens operation, and so on, and it is not easy to maintain and improve the number of shootings for a battery (unused battery or fully-charged battery). And therefore, there is a problem such that the number of shootings for the battery has to be reduced.
[Noise Contamination Due to Types of Imaging Element]
Generally, as imaging elements, solid-state imaging elements such as a CCD (charge-coupled device) image sensor and a CMOS (complementary metal-oxide semiconductor) image sensor are widely used. As for the CCD image sensor, an analog signal line to an A/D (analog-digital) converter is long, and on the contrary, as for the CMOS image sensor, the analog signal line to the A/D converter is short; therefore there is a characteristic such that the CMOS image sensor is hardly affected by external noise. However, the structure of the CMOS image sensor is such that an imaging section and an analog front end section (AFE) are incorporated, and there are many cases where a common power supply is used for power supply of a circuit section which operates inside the CMOS image sensor. As for a peripheral circuit of the CMOS image sensor, an individual provision of a power supply (for example, a power supply is individually provided with a ground pattern (GND) in an analog circuit and a ground pattern (GND) in a digital circuit, and/or a power supply is not commonalized, but is individually provided with a plurality of parts where the same power supply voltage is used, and so on.) is ideal; however there are many cases where the power supply is commonalized and designed due to miniaturization.